Console² replaces the JavaScript Console with what could be the next generation Error Console.

As of Firefox 1.5, the JavaScript Console is used not only for JavaScript errors, but for CSS errors as well. This extension takes this into account by providing a simple possibility to display errors by type. In case you find Console Filter too confusing, this might be what you’ve been looking for.

Console² let’s you display errors filtered by type (Errors, Warnings, Messages), language (JavaScript, CSS, XML) and context (Chrome, Content). Furthermore it provides a simple search box (as seen in the History and Bookmarks sidebars) and some accessibility improvements.

Read more at: Mozilla Forum